The invention relates generally to the manufacture of electronic devices. More specifically, this invention relates to photoresist overcoat compositions useful in photolithographic methods which allow for the formation of fine patterns using a negative tone development process.
In the semiconductor manufacturing industry, photoresist materials are used for transferring an image to one or more underlying layers, such as metal, semiconductor and dielectric layers, disposed on a semiconductor substrate, as well as to the substrate itself. To increase the integration density of semiconductor devices and allow for the formation of structures having dimensions in the nanometer range, photoresists and photolithography processing tools having high-resolution capabilities have been and continue to be developed.
Positive-tone chemically amplified photoresists are conventionally used for high-resolution processing using a positive tone development (PTD) process. In the PTD process, exposed regions of a photoresist layer become soluble in a developer solution, typically an aqueous alkaline developer, and are removed from the substrate surface, whereas unexposed regions which are insoluble in the developer remain after development to form a positive image. To improve lithographic performance, immersion lithography tools have been developed to effectively increase the numerical aperture (NA) of the lens of the imaging device, for example, a scanner having a KrF or ArF light source. This is accomplished by use of a relatively high refractive index fluid (i.e., an immersion fluid) between the last surface of the imaging device and the upper surface of the semiconductor wafer.
Considerable effort has been made to extend the practical resolution beyond that achieved with positive tone development from both a materials and processing standpoint. One such example is the negative tone development (NTD) process. The NTD process allows for improved resolution and process window as compared with standard positive tone imaging by making use of the superior imaging quality obtained with bright field masks for printing critical dark field layers. NTD resists typically employ a resin having acid-labile (also referred to herein as acid-cleavable) groups and a photoacid generator. Exposure to actinic radiation causes the photoacid generator to form an acid which, during post-exposure baking, causes cleavage of the acid-labile groups giving rise to a polarity switch in the exposed regions. As a result, a difference in solubility characteristics is created between exposed and unexposed regions of the resist such that unexposed regions of the resist can be removed by organic solvent developers, leaving behind a pattern created by the insoluble exposed regions.
Problems in NTD processing in the form of necking of contact holes and T-topping of line and trench patterns in the developed resist patterns are known and described in U.S. Application Pub. No. US2013/0244438A1. Such problems are believed to be caused by diffusion of stray light beneath edges of the photomask opaque pattern, undesirably causing polarity-switching in those “dark” regions at the resist surface. In an effort to address this problem, the '438 publication discloses use of a photoresist overcoat that includes a basic quencher, a polymer and an organic solvent. For purposes of immersion lithography, the '438 publication further discloses that the overcoat compositions can be used to form a barrier layer for avoidance of leaching of photoresist components into the immersion fluid and to provide desirable contact angle characteristics with the immersion fluid for increased exposure scan speeds.
There is a continuing need in the art for improved photolithographic methods and photoresist overcoat compositions for the formation of fine patterns with improved contact angle characteristics between the substrate surface and immersion fluid which would allow for greater immersion scanner speeds.